Gas turbine engines for aircraft are normally guaranteed for thrust performance. A newly manufactured engine must be tested to determine compliance with the guarantee. Precise measurement is required. Thrust testing is also required during engine development.
For sea level operation the testing may be performed outdoors. It is preferable however to use an indoor test cell which offers several advantages in performing the test. The test is less affected by variable wind and unreliable weather conditions. Noise also can be attenuated to avoid excessive disturbances.
With indoor testing for sea level performance, the engine is supported in a duct. The exhaust flow induces some engine bypass flow.
Where altitude testing is required it is necessary to operate an exhaust compressor to reduce the pressure within the test cell. Engine bypass air is not desirable since this excess air must now be pumped with an exhaust compressor. Accordingly inlet air guides are used to direct all of the inlet flow through the engine.
Discharge of the gas turbine engine exhaust into a pressure recovery pipe reduces the head against which the exhaust compressors must operate. It is conventional therefore to use this discharge pressure recovery pipe with a bulkhead between the pipe and the walls of the duct.
As the gas turbine engine discharge passes into this pressure recovery pipe some of the cell air is drawn into the pipe. Since there is no supply of air to the chamber, the air regurgitates out of the pipe, resulting in high velocity flows into the chamber. These high velocities are in the order of 30% of the engine jet velocity.
Pressure rise results as the high velocity is reduced. This results in an axially variable static pressure acting against the external surface of the engine. An axial thrust force is imposed against the engine resulting in a test measurement uncertainty of about 1%.
It is an object of the invention to reduce the velocity of the reverse flow external of the engine, whereby variations in static loading are reduced.